Question

A solenoid has an inductance of 60 H and a resistance of $30\Omega$. If it is connected to a 100 V battery, how long will it take for the current to reach $\frac{e-1}{e}=63.2\%$ of its final value?

• A. 1 s
• B. 2 s
• C. e s
• D. 2e s

Answer 1

The correct option is B 2 s

In a LR circuit, the time take to reach $63.3\%$ of the current is called the time constant and it is calculated as follows.
$\tau =\frac{L}{R}$
Therefore, $t=\frac{60}{30}=2seconds$.